The speed of communication between integrated circuits and subsystems has been increasing. It is often desirable to communicate at frequencies above a gigahertz (GHz). At these frequencies, the circuit lines must be treated as transmission lines (TLs) if good signal integrity and reliable transmission is required. Theoretically, it is ideal to terminate a TL in an impedance equal to its characteristic impedance (Z0). Z0 is normally expressed as the square root of the ratio of capacitance per unit length to inductance per unit length for a lossless TL.
Practical TLs are lossy, therefore, it is sometimes desirable to have the termination impedance higher than the characteristic impedance to cause a small reflection which adds to the transmitted signal in such a way to make the overall signal swing larger. The signals transmitted on TLs may lose their pure binary values and become analog. Receivers “detect” these analog signals and convert them back to binary signals with controlled logic one and zero levels. Receivers have threshold levels and a received signal that swings above the threshold level will cause the receiver output to be switched to a logic one and a signal that swings below the threshold will cause the receiver output to be switched to a logic zero. In practical circuits, receivers may not have threshold levels that are at the mid-point of the power supply. In these cases, it may be desirable to vary or change the offset of a received signal without changing the receiver termination impedance. Dynamically adjusting termination impedance and offset voltage would enable signal integrity and margins to be optimized.
There is, therefore, a need for a method and circuitry to allow the termination impedance and offset voltage to be varied independently to optimize signal transmission and reception.